As much importance has been attached to saving in resources and countermeasures to environment in recent years, requirements to reduce fuel consumption of automobiles have been increasingly severe. Automobile tires have also been required to low their rolling resistance so as to contribute to reduction in the fuel consumption. In order to low the rolling resistance of tires, a rubber material capable of providing vulcanized rubber having low heat build-up is generally used as a rubber material for the tires.
It has heretofore been proposed to use a rubber composition obtained by compounding silica as a reinforcing agent with a diene rubber in place of carbon black so as to reduce heat build-up. However, the silica-compounded rubber composition has involved a problem that it cannot exhibit sufficient abrasion resistance and tensile strength compared with the carbon black-compounded rubber composition. A cause for this problem is considered to be attributed to the fact that the affinity of the silica for the diene rubber is lower than that of the carbon black, so that a reinforcing effect cannot be sufficiently developed.
In order to enhance the affinity of the silica for the diene rubber, it has heretofore been proposed to use a silane coupling agent (Japanese Patent Application Laid-Open Nos. 252431/1991 and 252433/1991, etc.). However, this method requires to use an expensive silane coupling agent in a large amount for bringing about a sufficient effect.
The use of a diene rubber with a substituent having high affinity for silica introduced therein has been investigated as another improving method. For example, a diene rubber with a tertiary amino group (Japanese Patent Application Laid-Open No. 101344/1989) introduced therein has been proposed for a diene rubber obtained by an emulsion polymerization process. Besides, a diene rubber with an alkylsilyl group (Japanese Patent Application Laid-Open No. 188501/1989), a halogenated silyl group (Japanese Patent Application Laid-Open No. 230286/1993) or a substituted amino group (Japanese Patent Application Laid-Open No. 22940/1989) introduced therein has been proposed for a diene rubber obtained by an anionic polymerization process.
However, many of the diene rubbers with these substituents introduced therein have demerits that when silica is blended with these diene rubbers, they strongly aggregate with silica to form compounds having a high Mooney viscosity, so that the resulting diene rubber compositions become poor in processability because a failure in dispersion occurs, and that the various properties of the diene rubber, such as heat build-up resistance, tensile strength and abrasion resistance, are not sufficiently improved.